Embedded host on Minecraft port intermittently fails for new clients with Host server error

[GSstarGamer]

Bug description

AutoModpack’s embedded host on the Minecraft port intermittently fails for new clients during modpack download/install.

New players are disconnected with:

[AutoModpack] Host server error. Please contact server administrator to check the server logs!

This is not a full game/server crash. The Minecraft server itself stays online, but the AutoModpack download/host handshake fails for some fresh clients before
they can join.

What makes this unusual is that the same setup previously worked:

• On June 5-6, 2026, players successfully installed the modpack even when the server log showed Sending modpack host address: :-1
• On June 7, 2026, new players started failing repeatedly with Host server error
• During the failing period, the advertised host switched between :-1 and mc.rionnag.net:25565

Some clients can still complete has installed whole modpack, while others fail immediately, so the issue appears intermittent / client-path dependent rather than
a total outage.

Steps to reproduce

1. Run a dedicated Fabric 1.21.1 server with AutoModpack 4.0.5.

2. Use AutoModpack embedded host on the Minecraft port (bindPort = -1), with the server reachable at mc.rionnag.net:25565.

3. Have a fresh client with AutoModpack installed attempt to join the server and download the modpack.

4. In failing cases, the server log shows:

   • Hosting modpack on Minecraft port
   • Sending modpack host address: :-1 or sometimes mc.rionnag.net:25565
   • then Host server error. AutoModpack host server is down or server is not configured correctly

5. The client is disconnected before joining the game.

Note:
This does not happen 100% consistently for every player. Some players can still complete has installed whole modpack, while others fail repeatedly with the host
server error.

Expected behavior

A fresh client with AutoModpack installed should be able to connect, download/install the modpack from the embedded host on the Minecraft port, and then join the
server normally without being disconnected.

Actual behavior

Instead of downloading/installing the modpack successfully, some fresh clients are disconnected during the AutoModpack host/download step with:

[AutoModpack] Host server error. Please contact server administrator to check the server logs!

On the server side, the Minecraft server stays online, but the log shows:

• Hosting modpack on Minecraft port
• Sending modpack host address: :-1 or sometimes mc.rionnag.net:25565
• then Host server error. AutoModpack host server is down or server is not configured correctly

Some players still succeed and complete has installed whole modpack, while others fail repeatedly.

Relevant logs

Relevant logs uploaded to mclo.gs:

• Current failing latest.log: https://mclo.gs/N2SntU1
• June 7, 2026 rotated log showing mc.rionnag.net:25565 being advertised: https://mclo.gs/vtzU73E
• June 6, 2026 rotated log showing older successful installs while :-1 was advertised: https://mclo.gs/rjm4kwW

Minecraft & Mod Loader versions

Minecraft 1.21.1 Fabric Loader 0.16.14

Minecraft launcher

Modrinth App

Operating system

Windows 11

AutoModpack version

4.0.5

Other information

Server details:

• Dedicated Fabric server on a home network
• Minecraft is exposed directly through router port forwarding on mc.rionnag.net:25565
• No separate AutoModpack port is being used
• Embedded host is running on the Minecraft port (bindPort = -1)

Important observations:

• On June 5-6, 2026, AutoModpack successfully served installs while logging Sending modpack host address: :-1
• On June 7, 2026, the same general setup started failing for some new players with repeated Host server error
• On June 7, 2026, the advertised host also appeared as mc.rionnag.net:25565 in some logs, but some clients still failed
• The Java server is listening on port 25565
• DNS for mc.rionnag.net resolves correctly
• TCP connection to mc.rionnag.net:25565 succeeds
• Some players can still complete has installed whole modpack, while others fail repeatedly

I also tested pinning:

• addressToSend = "mc.rionnag.net"
• portToSend = 25565

So this does not look like a simple closed-port issue. It appears more like an embedded host / protocol issue on the Minecraft port.

Check list

• I have verified that the issue persists in the latest version of the mod.
• I have searched the existing issues and confirmed that this is not a duplicate.
• I have provided all the necessary information to reproduce the issue.
• I have verified that the issue does not occur without the AutoModpack - Ignore if you are reporting a mod conflict issue.